1. Field of Invention
The invention relates to the field of electronics and more particularly to the tuning and injection locking of voltage controlled oscillators (VCOs).
2. Description of the Related Prior Art
As will be understood by those skilled in the art, an oscillator is an electronic device used for the purpose of generating a signal. Oscillators are found in computers, audio-frequency equipment, particularly music synthesizers, and wireless receivers and transmitters. There are many types of oscillator devices, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback.
As will be also understood by those skilled in the art, some oscillators employ combinations of inductors, resistors, and/or capacitors to generate their operating frequency such as resistor-capacitor (RC) and inductor-capacitor (LC) oscillators. However, the best stability (constancy of frequency) is obtained in oscillators that use quartz crystals. When a direct current is applied to such a crystal, it vibrates at a frequency that depends on its thickness, and on the manner in which it is cut from the original mineral rock.
A voltage-controlled oscillator (VCO) is a circuit that generates an oscillating signal at a frequency proportional to an externally applied voltage. VCOs are often found in phase-locked loops (PLLs) used for, among other things, synchronizing an oscillation frequency to an external reference, or to a higher multiple or derivative of a crystal reference. In the first case (called clock recovery) the goal is to recreate a clock signal synchronous to that which was used to generate a data stream. In the second case, the goal is to generate a higher frequency signal that has the phase noise properties of a low frequency reference. The phase noise of a VCO is generally very poor and does not meet the need of many applications. It is also unpredictable in terms of it's nominal frequency and drift. The crystal on the other hand is much more exact and has better phase noise. To generate a high radio frequency (RF) using a lower frequency crystal oscillator as a reference, a PLL is used.
FIG. 1 depicts a block diagram of a frequency tuning circuit including a PLL with VCO circuit. The circuit consists of an oscillator 20 or external reference clock, a phase detector 22, a low-pass filter 24, a gain stage 26, a VCO 28 and a divider 29 configured in a loop. The phase detector is a circuit that normally has an output voltage with an average value proportional to the phase difference between the input signal from oscillator 20 and the output of VCO 28. The low-pass filter 24 is used to extract the average value from the output of the phase detector 22. The average value is then amplified and used to drive the VCO 28. The divider 29 transforms the high frequency Vosc into a low frequency V′osc. The negative feedback of the loop results in the output of the VCO 28 being synchronized or locked with the input signal from oscillator 20. In this locked condition, any slight change in the input signal first appears as a change in phase between the input signal and the VCO frequency. The phase shift then acts as an error signal to change the VCO frequency to match the input signal. In other words, the purpose of the phase detector 12 is to produce and output which represents how far the frequency produced in VCO 18 is from that of the input signal. Comparing these frequencies and producing an error signal proportional to their difference allows the VCO frequency to shift and become the same frequency as the input signal.
An alternate to PLL is injection locking which does not require the design of a high frequency phase detector or divider circuitry. The process of injection locking is a fundamental property of oscillators, in that it can be observed in a wide variety of oscillator types with the same qualitative behaviour observed in each case. When a periodic signal is injected into an oscillator (e.g. a VCO) with a free-running frequency f0, by summing it with the state oscillation signal, the oscillator will lock to and track the injected signal frequency over f0+/−LBW/2, where LBW is the locking bandwidth. Within this lock range, the process can be modeled as a true first-order PLL which implies that the loop is unconditionally stable and that the phase noise of the output tracks the phase noise of the injected clock over a wide bandwidth. Moreover, the locking bandwidth (LBW) increases for larger injection amplitudes. For example, to accommodate process and environment variations, typical radio frequency (RF) applications require an LBW of 25-50 MHz greater than the signal bandwidth. However, achieving this LBW typically requires a large injection signal power, which is undesirable in certain applications such as integrated (as opposed to discrete) circuits. Further, given the inherent advantages of digital (as opposed to analogue) signal processing, the ability to determine the oscillation frequency of the VCO using a digital circuit would be advantageous. In particular, in an integrated circuit environment, the ability to eliminate off-chip components and to reduce the overall chip area is desirable.